The present invention relates to helicopters, and, in particular, relates to a system for detecting cracks in helicopter blades that have hollow spars and are either pressurized or have a vacuum therein.
Helicopter blades are very long and slender as compared to typical aircraft blades and are thus subject to severe stress from flexing, bending, twisting, etc. which considerably shortens the blade's life from fatigue. Hollow blades being lighter are subject to greater stress because they are typically used on military helicopters which are more prone to violent maneuvers. It is clearly obvious that any pending failure in the blade evidenced by cracks should be detected as soon as possible to prevent catastrophic failure.
X-ray pictures and magnetic flux preventive maintenance procedures can be performed after so many designated hours of use; however, x-rays will not detect a crack less than about half an inch and magnetic flux will not work on non-magnetic materials such as aluminum and titanium, for example. Therefore, the pressurized or evacuated spar is used to detect cracks by means of a differential pressure change in the hollow spar.
In order to detect cracks in rotor blades before failure several prior systems have been used.
One prior system called the Blade Inspection Method (BIM) uses pressure switches in each blade that indicate if the pressure within the spar falls below a preset level A signal from the rotating switch is transferred through a set of slip rings attached at the base of the rotor shaft to a warning lamp in the cockpit. One problem occurred when the blade stopped rotating then the pressure switch would return to its normal state as a result of the air pressure equalizing in the blade. Another problem occurs in the slip rings. The slip rings become worn and dirty and may not pass a critical signal with any reliability.
This system has been replaced in many helicopters by the Inflight Blade Inspection System (IBIS) In this system, the pressure sensor device, when activated by a gas pressure change, exposed a radioactive source such as Sr-90 to the atmosphere. The radiation is detected by a Geiger-Mueller tube assembly that is mounted on the airframe near the rotating blades. See, for example, U.S. Pat. Nos. 3,985,318 and 4,727,251 which are incorporated by reference. This system is not resistant to electromagnetic interference (EMI) and results in false alarms when the helicopter is flown near radars. Retrofit for EMI hardening is very expensive. Also, the nuclear isotopes must be replaced at about 10 years which is also expensive for a large fleet. The use of nuclear material further requires special handling, permits, licensing, shipping, repairs and disposal procedures, all time consuming and expensive.
Other prior systems use an induction coil or a conductive wire, or pressure sensitive transducer, for example, to detect cracks. This information is transmitted to the aircraft by, for example, inductive coil coupling.
U.S. Pat. Nos. 4,026,660; 4,106,332; 4,345,237 and 4,524,620 are incorporated by reference.
Therefore, there is a need for a crack detection system that is reliable, easily maintained, inexpensive and can be retrofitted into existing blades without modification thereto.